Patent document 1 below discloses a surge protection circuit in a three-phase four-wire circuit, which is coupled between a system side and a load side. The surge protection circuit disclosed in patent document 1 includes surge absorbing elements such as varistors coupled between the R-phase and the neutral phase (N-phase), the S-phase and the N-phase, and the T-phase and the N-phase. Normally (that is, when no lightning surges occur), there is no conduction through the surge absorbing elements. When a lightning surge occurs, a high voltage of the lightning surge causes conduction through the surge absorbing elements, and the lightning surge is returned to the system side. Thus, the load side is protected against the lightning surge.